384 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



the thickening at the joints of a tubular stem in which the transverse 

 diaphragm is not completed, as in some umbelHfers. In the case of 

 iron tubing, the entire length is subsequently filled with concrete. 



The diaphragm is more or less cup-shaped, or in the form of an 

 inverted cone. This is the strongest mechanical arrangement for 

 resisting the tendency to tear the internode above it. For the " pull " 

 upwards, if the wind is trying to break it across, is met by a corre- 

 sponding pull downwards. The force is thus neutralized, if it can 

 resist the strain. 



Another method of strengthening an iron angular column is by the 

 addition of " angle-irons " at the corners, to which they are adapted 

 inside. This corresponds to the collenchymatous strands of the four 

 corners of the quadrilateral stems of Galiums and the dead-nettle. 



The common heath-grass (Molinia) shows an additional method of 

 supplying strength, though hollow in the centre, namely by a zone of 

 strong fibrous tissue in which the vascular cords are imbedded. 



Mechanics of Climbers. — Tropical woody climbers or lianes are 

 particularly interesting in consequence of the various mechanical 

 contrivances adopted to enable them to resist the enormous strains to 

 which they must be submitted, as well as their own weight. A simple 

 case is seen in the genus Bauhinia. The stem at first is normal or 

 cyhndrical, but it soon becomes flattened by a greater growth being 

 made along one diameter. It then becomes ribbon-like and more or 

 less flexible, but strong. It also introduces the " cup or arch " mechan- 

 ism on alternate sides ; so that every cup or arch stands in the bases of 

 two others. 



The horizontal force of the wind or other strain acting laterally at 

 any point is effectively met and neutralized in a similar way to that of 

 the cup-like diaphragm of the bamboo. 



A corrugated iron sheet is made on much the same principle, so 

 that if a transverse strip be cut off it would be something Hke the 

 stem of Bauhinia, only the surface being simply flat instead of 

 " cupped." Of course the corrugated or " arch upon arch " structure, 

 as it might be called, adds materially to the strength of sheet iron in 

 resisting strains. Sometimes a long brick wall for fruit trees is built 

 in a series of curves or a sinuous manner. It can then be made of less 

 material than a straight wall would require. 



In the gigantic climber called the " monkey-ladder " (Caulotretus) 

 the same kind of mechanism exists, but additional strength is acquired 

 by flanges being added to the edges of the stem. These, of course, 

 greatly stiffen the stem and make the whole more rigid, hke a girder. 



Two more examples of " corrugations " may be mentioned. Boiler 

 flues (within circular tubes) are made with a succession of rings and 

 depressions, the object being to produce greater strength to resist the 

 pressure of steam within them. 



Bridges &c. are often built on a series of angular arches of iron, each 

 arch being in the form of a " frustum," or truncated cone, or p5Tamid, 

 erect and inverted successively. Great strength is thus acquired for 



